The field or art to which the invention pertains includes an adjustable open-end wrench or pipe wrench. Conventionally, an open-end wrench has been used to turn a bolt or nut of hexagonal or rectangular shape having two opposite sides or a similar periphery or to tighten or release a coupling or the like, but has been incapable of gripping a pipe and the like having a circular periphery since its jaws are fixed for a certain size. Therefore, to grip and rotate objects having a circular periphery, a pipe wrench is exclusively employed. The pipe wrench, however, is unsuitable for turning a bolt or nut since the adjustable jaw of the pipe wrench is designed to bite the surface of an object to be turned. Particularly, when turning not only bolts and nuts but pipes, a crescent wrench and a pipe wrench must be exchanged frequently. This is very cumbersome. Therefore, the demand for an adjustable wrench which can be used both as a crescent wrench and a pipe wrench has been strong.